Japanese unexamined patent application publication No. JP-A-2007-101636 (Hereinafter, Patent document 1) describes a first related art image forming apparatus. The first related art image forming apparatuses is an electro-photographic color printer such as a so-called tandem type image forming apparatus in which photosensitive drums corresponding to respective colors of yellow, magenta, cyan and black are disposed in parallel.
In such a tandem type image forming apparatus, developing rollers are provided so as to be associated with the respective photosensitive drums. The photosensitive drums and the developing rollers are driven to rotate in such a state that the developing rollers are kept in contact with the corresponding photosensitive drums. Thus, toner is supplied to latent images formed on the photosensitive drums by static electric charges imparted thereto from the developing rollers, whereby toner images are carried on the photosensitive drums. Thus, toner images of colors which correspond to the respective colors of the photosensitive drums are formed on the photosensitive drums, and the toner images of the respective colors are transferred sequentially onto a sheet passing underneath the photosensitive drums in an overlapped fashion, whereby the formation of a color image on the sheet is attained. Alternatively, a black toner image is formed only on the photosensitive drum of black, and the black toner image is transferred onto a sheet, whereby the formation of a monochrome image on the sheet is attained.
When a monochrome image is formed, no toner image is formed on the photosensitive drums other than the photosensitive drum of black, that is, on the photosensitive drums of yellow, magenta and cyan. Thus, it is advantageous if the corresponding developing rollers are spaced apart from the photosensitive drums of yellow, magenta and cyan so as to prevent the wear of the developing rollers.
Accordingly, there has been proposed a tandem type image forming apparatus in which a connecting-disconnecting member is provided which is movable in a straight line in a direction in which photosensitive drums are aligned, and the photosensitive drums are switched by the connecting-disconnecting member so as to be put in an all color photosensitive drums disconnected state in which developing rollers are disconnected and spaced apart from their mating photosensitive drums, a black photosensitive drum in-contact state in which the developing roller of the black photosensitive drum is brought into contact with its mating photosensitive roller, while the developing rollers of the yellow, magenta and cyan photosensitive drums are spaced apart from their mating photosensitive rollers, and an all color photosensitive drums connected state in which the developing rollers of all the photosensitive drums are brought into contact with their mating photosensitive rollers.
Photosensitive drums become deteriorated as they are used, and the deteriorated photosensitive drums have to be replaced with new ones. When photosensitive drums are made to be detachably attached to an apparatus main body of an image forming apparatus, the replacement of the deteriorated photosensitive drums with new ones can be attained easily.
When photosensitive drums are made to be detachably mounted in the apparatus main body, however, in the event that the photosensitive drums are mounted in positions deviating from normal positions, or that the photosensitive drums are caused to shift from the normal positions, there is caused an error in forming an image in a proper position on a sheet. That is, according to the related art apparatus described in the Patent Document 1, a mechanism is necessary for fixing the photosensitive drums in the proper positions, and this causes a disadvantage that the configuration of the image forming apparatus becomes complex.
Further, Japanese unexamined patent application publication No. JP-A-2000-250310 (Hereinafter, Patent document 2) describes a second related art image forming apparatus. The second related art image forming apparatus is an electro-photographic printer such as an apparatus in which process cartridges are detachably mounted in an apparatus main body.
In a related art image forming apparatus of this type, for example, pairs of left and right guide units are provided within a body housing. Photosensitive drums are respectively provided in the process cartridges. Each of the photosensitive drums is held in a housing of its respective process cartridge in such a state that both end portions of a shaft of the photosensitive drum project outwardly sideways from both side walls of the housing. When mounting the process cartridges in the body housing, the process cartridges are inserted into the body housing while both the end portions of the shafts of the photosensitive drums (drum shafts) are being guided by the guide units. Then, when the end portions of the drum shafts are brought into abutment with predetermined portions of the guide units, the movement of the process cartridges is restricted, whereby the mounting of the process cartridges in the body housing is attained.
However, according to the related art apparatus described in the Patent Document 2, when mounting process cartridges in a body housing of a printer, in some cases, some of the process cartridges are inserted into the body housing with force. As this occurs, both end portions of drum shafts are brought into abutment with predetermined portions of guide units with force, whereby impact is imparted to both the end portions of the drum shafts and the predetermined portions of the guide units. In the event that such impact is imparted repeatedly to the end portions of the drum shafts and the predetermined portions of the guide units while the process cartridges are mounted and dismounted repeatedly in and from the body housing, there is caused a risk that the predetermined portions of the guide units are deformed or damaged or portions of housings of the process cartridges where the drum shafts are inserted wear down, whereby there occurs a minute error in properly positioning the photosensitive drums.